1. Field of the Invention
This invention relates to the manufacture of polysulfide compositions of the type used, for example, as sealants in glazing and in other applications in the construction industry.
2. Description of the Prior Art
In preparing polysulfide sealants, a mercaptan-terminated polymer, generally a liquid polysulfide of relatively low molecular weight, is reacted with a curing agent, preferably an oxidising agent such as a metal peroxide or sodium perborate, to cure it to a solid state. The preferred liquid polysulfide polymers are those having a molecular weight of 1000-8000, a viscosity of 1 to 200 Pas and a degree of crosslinking of 2.0 or less. The preferred polymers are those prepared by Morton International Inc. and known as LP-2, LP-32 etc. The chemistry embodied in this reaction applies to all mercaptan terminated liquid polymers. ##STR1##
Curing agents such as calcium peroxide require the presence of water to activate them. It is therefore possible, instead of using a cumbersome two-part sealant composition, to use a one-part composition of liquid polysulfide and curing agent which is kept free of water until used. Once the sealant is in place it forms a skin and slowly cures to a rubber state through the presence of atmospheric moisture.
EP-A-0 377 485 discloses a one-part polysulfide sealant composition which is partially cured and in which curing is stopped by the addition of a desiccant to give a stable prepolymer which can be stored and subsequently cured, without an induction period, by exposure to moisture and optionally a further curative.
The cure rate of a one-part sealant composition, which is generally measured in terms of tack-free time and speed of through-cure, is controlled to a large extent by the overall alkalinity of the formulation. High alkalinity enhances the cure rate, and it is common practice to increase the alkalinity by adding oxides of heavy metals, especially barium oxide, BaO.
Barium oxide has a dual role in one-part polysulfide compositions, and is primarily added as a desiccant, to absorb all the water from the composition and thus inhibit chemical curing to enable the composition to be stored. This can be done after an initial curing period, as in EP-A-0 377 485, since the initial cure does not significantly increase viscosity but enables the subsequent full cure to be effected without an induction period. The composition, which may contain a cure catalyst such as calcium peroxide, can be stored in suitable watertight containers such as drums, tins or plastic cartridges, for a year or more before use.
Upon exposure of the sealant composition the barium oxide absorbs atmospheric moisture and becomes saturated, after which water is available to activate the catalyst and initiate the final cure. The hydrated oxide is highly alkaline, especially in the case of barium oxide, and the cure proceeds rapidly on the surface, giving rise to a rapid loss of tackiness.
The use of barium oxide has two disadvantages. First, BaO is toxic with the result that the composition has to be labelled as toxic. Secondly, its overall effect on the cure rate is a negative one because of its efficacy as a desiccant. If the BaO content is reduced below the level at which the composition has to be labelled as toxic, the tack-free time becomes unacceptably long, even though the through-cure may proceed more rapidly.